Determine if uploaded file is image (any format) on MVC

In case it can helps anyone, Here is a static method for HttpPostedFileBase that checks if a given uploaded file is an image:

public static class HttpPostedFileBaseExtensions
{
    public const int ImageMinimumBytes = 512;

    public static bool IsImage(this HttpPostedFileBase postedFile)
    {
        //-------------------------------------------
        //  Check the image mime types
        //-------------------------------------------
        if (!string.Equals(postedFile.ContentType, "image/jpg", StringComparison.OrdinalIgnoreCase) &&
            !string.Equals(postedFile.ContentType, "image/jpeg", StringComparison.OrdinalIgnoreCase) &&
            !string.Equals(postedFile.ContentType, "image/pjpeg", StringComparison.OrdinalIgnoreCase) &&
            !string.Equals(postedFile.ContentType, "image/gif", StringComparison.OrdinalIgnoreCase) &&
            !string.Equals(postedFile.ContentType, "image/x-png", StringComparison.OrdinalIgnoreCase) &&
            !string.Equals(postedFile.ContentType, "image/png", StringComparison.OrdinalIgnoreCase))
        {
            return false;
        }

        //-------------------------------------------
        //  Check the image extension
        //-------------------------------------------
        var postedFileExtension = Path.GetExtension(postedFile.FileName);
        if (!string.Equals(postedFileExtension , ".jpg", StringComparison.OrdinalIgnoreCase)
            && !string.Equals(postedFileExtension , ".png", StringComparison.OrdinalIgnoreCase)
            && !string.Equals(postedFileExtension , ".gif", StringComparison.OrdinalIgnoreCase)
            && !string.Equals(postedFileExtension , ".jpeg", StringComparison.OrdinalIgnoreCase))
        {
            return false;
        }

        //-------------------------------------------
        //  Attempt to read the file and check the first bytes
        //-------------------------------------------
        try
        {
            if (!postedFile.InputStream.CanRead)
            {
                return false;
            }
            //------------------------------------------
            //   Check whether the image size exceeding the limit or not
            //------------------------------------------ 
            if (postedFile.ContentLength < ImageMinimumBytes)
            {
                return false;
            }

            byte[] buffer = new byte[ImageMinimumBytes];
            postedFile.InputStream.Read(buffer, 0, ImageMinimumBytes);
            string content = System.Text.Encoding.UTF8.GetString(buffer);
            if (Regex.IsMatch(content, @"<script|<html|<head|<title|<body|<pre|<table|<a\s+href|<img|<plaintext|<cross\-domain\-policy",
                RegexOptions.IgnoreCase | RegexOptions.CultureInvariant | RegexOptions.Multiline))
            {
                return false;
            }
        }
        catch (Exception)
        {
            return false;
        }

        //-------------------------------------------
        //  Try to instantiate new Bitmap, if .NET will throw exception
        //  we can assume that it's not a valid image
        //-------------------------------------------

        try
        {
            using (var bitmap = new System.Drawing.Bitmap(postedFile.InputStream))
            {
            }
        }
        catch (Exception)
        {
            return false;
        }
        finally
        {
             postedFile.InputStream.Position = 0;
        }

        return true;
    }
}

Edit 2/10/2017: According to a suggested edit, added a finally statement to reset the stream, so we can use it later.

It's 2018 and the accepted answer does not work with .NET CORE 2.1 because we now have IFormFile instead of HttpPostedFileBase.

Here comes the adaption of the accepted answer to .NET CORE 2.1 (I also fixed the bug/typo mentioned by TomSelleck in his comment to the accepted answer):

public static class FormFileExtensions
{
    public const int ImageMinimumBytes = 512;

    public static bool IsImage(this IFormFile postedFile)
    {
        //-------------------------------------------
        //  Check the image mime types
        //-------------------------------------------
        if (postedFile.ContentType.ToLower() != "image/jpg" &&
                    postedFile.ContentType.ToLower() != "image/jpeg" &&
                    postedFile.ContentType.ToLower() != "image/pjpeg" &&
                    postedFile.ContentType.ToLower() != "image/gif" &&
                    postedFile.ContentType.ToLower() != "image/x-png" &&
                    postedFile.ContentType.ToLower() != "image/png")
        {
            return false;
        }

        //-------------------------------------------
        //  Check the image extension
        //-------------------------------------------
        if (Path.GetExtension(postedFile.FileName).ToLower() != ".jpg"
            && Path.GetExtension(postedFile.FileName).ToLower() != ".png"
            && Path.GetExtension(postedFile.FileName).ToLower() != ".gif"
            && Path.GetExtension(postedFile.FileName).ToLower() != ".jpeg")
        {
            return false;
        }

        //-------------------------------------------
        //  Attempt to read the file and check the first bytes
        //-------------------------------------------
        try
        {
            if (!postedFile.OpenReadStream().CanRead)
            {
                return false;
            }
            //------------------------------------------
            //check whether the image size exceeding the limit or not
            //------------------------------------------ 
            if (postedFile.Length < ImageMinimumBytes)
            {
                return false;
            }

            byte[] buffer = new byte[ImageMinimumBytes];
            postedFile.OpenReadStream().Read(buffer, 0, ImageMinimumBytes);
            string content = System.Text.Encoding.UTF8.GetString(buffer);
            if (Regex.IsMatch(content, @"<script|<html|<head|<title|<body|<pre|<table|<a\s+href|<img|<plaintext|<cross\-domain\-policy",
                RegexOptions.IgnoreCase | RegexOptions.CultureInvariant | RegexOptions.Multiline))
            {
                return false;
            }
        }
        catch (Exception)
        {
            return false;
        }

        //-------------------------------------------
        //  Try to instantiate new Bitmap, if .NET will throw exception
        //  we can assume that it's not a valid image
        //-------------------------------------------

        try
        {
            using (var bitmap = new System.Drawing.Bitmap(postedFile.OpenReadStream()))
            {
            }
        }
        catch (Exception)
        {
            return false;
        }
        finally
        {
            postedFile.OpenReadStream().Position = 0;
        }

        return true;
    }
}

For anyone that runs into this.

You could also use a file.ContentType.Contains("image") to check if the content type is of image/*.

if(file.ContentLength > 0 && file.ContentType.Contains("image"))
{
    //valid image
}
else
{
    //not a valid image
}

Not sure if this is best practice, but it works for me.